Single-wall carbon nanotubes (SWNT), commonly known as “buckytubes,” have unique properties, including high strength, stiffness, thermal and electrical conductivity. SWNT are hollow, tubular fullerene molecules consisting essentially of sp2-hybridized carbon atoms typically arranged in hexagons and pentagons. Single-wall carbon nanotubes typically have diameters in the range of about 0.5 nanometers (nm) and about 3.5 nm, and lengths usually greater than about 50 nm. Background information on single-wall carbon nanotubes can be found in B. I. Yakobson and R. E. Smalley, American Scientist, Vol. 85, July–August, 1997, pp. 324–337 and Dresselhaus, et al., Science of Fullerenes and Carbon Nanotubes, 1996, San Diego: Academic Press, Ch. 19.
Single-wall carbon nanotubes are generally made in high-temperature processes using a carbon source and a metallic catalyst, typically comprising Group VIb and/or Group VIIIb transition metals. Methods for synthesizing single-wall carbon nanotubes include DC arc processes; laser vaporization of graphite doped with transition metal atoms; high temperature, high pressure gas-phase syntheses involving a carbon-containing feedstock gas, such as carbon monoxide; and a volatile transition metal catalyst precursor, and chemical vapor deposition (CVD) processes in which single-wall carbon nanotubes are formed from a carbon-containing gas on nanometer-scale metal catalyst particles, which can be supported on a substrate or catalyst support.
All known methods of synthesizing single-carbon nanotubes also produce a distribution of reaction products, including, but not limited to, single-wall carbon nanotubes, amorphous carbon, metallic catalyst residues, and, in some cases, multi-wall carbon nanotubes. The distribution of reaction products will vary depending on the process and the operating conditions used in the process. In addition to the distribution of reaction products, the process type and operating conditions will also produce single-wall carbon nanotubes having a particular distribution of diameters and lengths. Generally, the lengths of as-produced single-wall carbon nanotubes are in excess of about 50 nm, and more typically, greater than about 100 nm. Commonly, single-wall carbon nanotubes have lengths in the range of about 1 and about 10 microns.
Short lengths of nanotubes are often useful in various applications, such as, in flat panel displays, in composites, and as “seeds” for further nanotube growth. These short lengths are not economically or conveniently obtained from known single-wall carbon nanotube production processes, since the as-synthesized nanotubes are usually much longer than desired, and, in many cases, very entangled or bundled. Attempts to cut or shorten single-wall carbon nanotubes are complicated by two major factors. First is the nanotubes' extreme lack of solubility in water and most common solvents. Second is the strong propensity of single-wall carbon nanotubes to “rope” together in bundles that are strongly held together by van der Waals forces. The roping phenomenon produces aggregates of single-wall carbon nanotubes arranged together in aligned bundles or “ropes”. These aggregates are very cohesive, such that a pair of single-wall carbon nanotubes contacting one another along their lengths can have a tube-to-tube binding energy of up to about 500 eV/micron.
Methods for shortening or cutting single-wall carbon nanotubes have included chemical means, such as by oxidative acid treatment; physical methods, such as physical abrasion and sonication; and combinations thereof. One method for chemically “shortening” the single-wall carbon nanotubes is based on the oxidation of SWNT using a mixture of concentrated sulfuric and nitric acids. (See International Patent Publication WO 98/39250, “Carbon Fibers Formed from Single-Wall Carbon Nanotubes,” published Sep. 11, 1998, and Liu et al., Science 280, (1998) p.1253, both of which are incorporated herein by reference.) Physical means can also be used to shorten or cut single-wall carbon nanotubes. Examples of physical means for cutting nanotubes include, but are not limited to, abrasion, such as described in G. Maurin, et al., “Segmented and opened multi-walled carbon nanotubes,” Carbon 39 (2001), pp. 1273–1287, sonication, such as described in K. B. Shelimov, et al., “Purification of single-wall carbon nanotubes by ultrasonically assisted filtration,” Chem. Phys. Lett., 282 (1998) pp. 429–434, and, application of an electric voltage, such as described by A. Rubio, et al., “A mechanism for cutting carbon nanotubes with a scanning tunneling microscope,” Eur. Phys. J. B, 17 (2000) pp. 301–308. Another means of cutting single-wall carbon nanotubes can include both chemical and physical means. An example of a combination of means for cutting single-wall carbon nanotubes, using sonication and homogenization of a mixture of single-wall carbon nanotubes and polymethylmethacrylate in a monochlorobenzene solution, is described in Yudasaka et al., Appl. Phys. A 71, (2000) pp. 449–451.